The present invention relates to an apparatus for drying and heating material in a rotary kiln by means of regenerative heat transfer, and especially in kilns for reburning lime sludge.
Lime sludge is a term for a calcium carbonate sludge generated in a causticizer of a sulphate cellulose factory. This sludge is dried mechanically in a drum filter or in a drainage press before it is regenerated into calcium oxide in a rotary kiln.
Lime sludge from a drum filter usually has a 25-40% water content of the wet weight. The remaining water is removed by means of hot flue gases in a lime sludge reburning kiln. In order to make the heat transfer more effective, drying chains are usually used in the kiln. The chains function on a regenerative heat transfer principle storing heat from the gases and transmitting the heat to the lime sludge as the chains alternately contact the gases and the lime sludge, respectively, while the kiln is rotating around its axis.
A zone of chains has proved to be an effective means for heat transfer. It has, however, some disadvantages. For example, it is not advantageous to totally dry the lime sludge in the zone of chains, because dust begins to form in the kiln. Substantial quantities of the lime sludge may thus be disadvantageously discharged with the gas from the kiln. The development of drum filters for drying lime sludge mechanically has resulted in an increased dry solids content of the lime sludge before its entry into the kiln. Approximately 10 years ago, a typical dry solids content of lime sludge, after passage through the drum filter, was 65-70%. Today, however, it is an everyday phenomena to exceed a 70% dry solids content, and the best levels for the dry solids content are close to 80%. As noted previously, however, there is increased dust content in the discharge gases of the kiln caused by the action of the chains in the chain zone. When the kiln rotates, the chains swing and strike against each other and the almost completely dried lime sludge which clings to the chains comes loose in the flue gas atmosphere and is discharged with the flue gas.
The rest of the drying of the lime sludge takes place in the kiln part downstream of the chain zone. This part of the kiln usually has no inside protuberances or inwardly extending projections. The steel housing of the kiln is normally protected along its inside wall by means of a fire-resistant and isolating lining. Heat is transferred from the gas to the lime sludge bed surface in contact with the gas (i.e., the lime sludge bed surface/flue gas interface) and via the lined inner surface of the kiln to the lower bed surface contacting the lining (i.e., the bed bottom/flue gas interface). Heat is primarily transferred by radiation, but also in part by convection.
Dry lime sludge is finely powdered and very light (the density of calcium carbonate is about 2.7 t/m.sup.3, the bulk density of dry lime sludge is only 0.7-0.8 t/m.sup.3). The friction between the lime sludge bed and the lining, as well as the interior friction of the materials of the bed, are very small. Consequently, the bed behaves almost like water. The bed slips against the lining and its surface cannot renew. Because of this, the lime sludge surface that is contacting the gas and also its surface in contact with the lining are heated to a temperature that is considerably higher than the average temperature of the bed. This is also partly due to a low thermal conductivity resulting from the "porousness" of the bed. This situation obtains until the alkali salts start melting and forming grain. A grainy bed behaves normally in the kiln: the bulk density of the bed increases, the friction of the bed becomes sufficiently large to renew the surface of the bed and the temperature of the bed decreases. Heat transfer from the gas becomes more effective and the mixing of the bed takes place with sufficient effectiveness. This bed temperature at which the situation starts to improve is about 500.degree. C.
Lifters, for example, scoops, fixed on the inner periphery as in known drum dryers and some other kilns (e.g., expanded clay kilns) have lately been used in lime sludge kilns. By means of these lifters, the bed is mixed and its surface allowed to renew. Some sludge, however, adheres to these scoops depending on their configuration. When each scoop is elevated to its uppermost position in the kiln due to the rotation of the kiln, lime sludge falls from the scoop into the flue gas and thus adds to the formation of dust. Also, the mass of the scoops is usually so small that regenerative heat transfer is not as efficiently accomplished with them as is with drying chains.
According to the present invention, there is provided an apparatus for heating and drying material, for example, lime sludge, in a rotary kiln which functions on a regenerative heat transfer principle and at the same time mixes the bed effectively so that the surface is continuously renewed. It also provides more even temperature distribution throughout the bed and reduces the formation of dust and hence the quantity of material discharged with the flue gas. Such apparatus, according to the present invention, may comprise a housing and a plurality of rods disposed within the housing and spaced therefrom and one from the other. Preferably, the apparatus comprises rods that are circumferentially spaced one from the other and generally parallel with the longitudinal axis of the cylindrical housing whereby the rods are in alternate contact with the gas and lime sludge bed while the kiln rotates. When in the bed, the rods advantageously and effectively mix it. When in the gas, the preferred round form of the rods inhibits any scooping action and permits only a small amount of such lime sludge to adhere to the rods. Thus, when the sludge falls from the rods, only a minimum amount becomes mixed with the gas and only minimally adds to the formation of dust in the kiln. The effect of the regenerative heat transfer is primarily dependent on the mass of the rods. It is believed possible to provide a rod weight per kiln meter as large as the weight of the chain zone without incurring the deleterious effects of the chains noted previously.
According to the present invention, the circumferentially-spaced axially extending rods lie in an annular region extending from the interior wall of the housing inwardly toward the central axis a distance at least 15% but no greater than 40%, and preferably 30%, of the diameter of the housing. While the preferred form of the rods is that of a solid circular cross-section, other shapes are possible, for example, rods having a square or droplet shaped cross-section may be used. In those latter cases, the orientation of the rods is in a direction to facilitate movement of the rods through the bed with minimum adhesion of the material to the rods consistent with good regnerative heat transfer characteristics between the rods and the material. Thus, the tips or apices are oriented or faced in the direction of rotary movement of the rods through the bed.